The present invention relates to a power distribution control apparatus for a four-wheel drive vehicle.
Four-wheel drive vehicle systems include full time drive systems, part-time drive systems, and on-demand drive systems (torque split drive systems). A full-time drive system constantly drives four wheels. A part-time drive system switches between four-wheel drive and two-wheel drive as needed. An on-demand drive system performs a seamless transition between four-wheel drive and two-wheel drive as needed. A four-wheel drive vehicle equipped with a full time drive system has a center differential between the front wheels and the rear wheels. Since the center differential permits the front wheels and the rear wheels to rotate at different rotation speeds, full-time four-wheel drive is possible.
Part-Time Drive System
As a four-wheel drive vehicle equipped with a part-time drive system, a vehicle in which the drive mode is switched from two-wheel drive to four-wheel drive is known. In such a four-wheel drive vehicle, computations are performed in which the lateral acceleration, the steering speed, the throttle opening degree, and the speed of increase of the throttle opening degree are each multiplied by a predetermined coefficient. Based on the computation results, the degree of sporty driving behavior on the part of the driver is computed. In other words, how briskly the driver manipulates the vehicle is detected. Then, based on the computed degree of sporty driving, the drive mode is switched between the two-wheel drive and the four-wheel drive (for example, refer to Japanese Laid-Open Patent Publication No. 10-272955).
On-Demand Drive System
A four-wheel drive vehicle equipped with an on-demand drive system includes main drive wheels that are directly coupled to the engine and auxiliary drive wheels (follower wheels) that are coupled to the engine with a power transmitting device. According to the road condition and the driving state, the coupling force (engaging force) of the power transmitting device is varied to optimize the power distribution to the auxiliary drive wheels. The power transmitting device has an electromagnetic clutch mechanism. By controlling the current to the electromagnetic coil incorporated in the electromagnetic clutch mechanism, the clutch discs are engaged with one another so that the housing and the rotary shaft are coupled to each other. Accordingly, the power generated by the engine is transmitted to the auxiliary drive wheels. Specifically, a wheel speed sensor is provided in each drive wheel. Based on detection results of the wheel speed sensors, the difference between the average speed of the main drive wheels and the average speed of the auxiliary drive wheels is obtained. The current to the electromagnetic coil is controlled based on the difference of the average speeds. Accordingly, the vehicle is driven in the four-wheel drive mode. For example, when the main drive wheels or the auxiliary drive wheels skid, the power transmitting device is electronically controlled to perform the four-wheel drive.
That is, in the four-wheel drive vehicle of the on-demand system, the power transmitting device controls the engaging force of the clutch discs. Accordingly, one of a first drive mode and a second drive mode is selected and the power distribution ratio to the front wheels and the rear wheels are controlled. In the same driving state, the power distribution to the front wheels and the rear wheels is closer to the equalized state in the second drive mode than in the first drive mode.
The four-wheel drive vehicle of the part-time type disclosed in the above publication only has two drive modes, that is, the two-wheel drive and the four-wheel drive. Therefore, the ratio of the power distribution cannot be properly controlled in accordance with the driving state.
In the case of a four-wheel drive vehicle of the on-demand system, if, for example, the driver prefers sporty driving, the drive mode is switched from the first drive mode to the second drive mode to enhance the performance of the vehicle. Therefore, the driving behavior of the driver is determined based on various parameters such as the vehicle speed, the angle of the steering wheel, and the angular velocity of the steering wheel. Based on the result of the determination, the drive mode is switched to the second drive mode. However, the driving behavior of the driver cannot be accurately detected with these parameters in some cases. Thus, the drive mode sometimes cannot be appropriately switched from the two-wheel drive to the four-wheel drive in some cases. Therefore, the ratio of the power distribution may fail to be properly controlled in accordance with the driving state.